You have £13.40 to loose if it doesn't fix your problem or make a big enough improvement in performance that you end up changing the Citroen C3 1.4 Diesel Catalytic Converter (with one from eBay?) anyway (£92+fitting).
Its your choice, based on how you feel about spending more money on the car. I do hope the guy that looked at the car this time is honest. A clogged cat, a choked exhaust silencer or any part of the exhaust system being restricted can give performance problems.
Some useful tips for 'using cataclean on your C3'
https://www.systemproducts.com/faq.html wrote: How do I maximize the results of Cataclean?
Follow these simple instructions:
1. Make certain that you have ¼ of a tank of fuel in your tank (3-4 gallons)
2. Allow vehicle to reach full temperature before introducing Cataclean.
3. Pour entire contents (16.7 oz) of Cataclean into the fuel tank.
4. Drive vehicle for 10 -15 miles